With continuous increases in the price of gold in both world and domestic markets, manufacturers of metals used in dentistry have commenced to market non-precious and semi-precious metals as a substitute for high gold content precious metals. These metal alloys materially reduce the cost of raw material, and ultimately, the cost of the completed porcelain-to-metal restoration. The difference in cost of new material can amount to as much as $150 to $200 per ounce. As approximately twelve to fifteen prosthetic teeth can be created per ounce of gold, the effective savings in using cheaper metals will be readily appreciated.
However, unlike gold, the substitute metals are far more chemically reactive than noble metals, and during usual baking procedures involved in the application of multiple coats of porcelain to the prosthetic teeth, gaseous reaction products are formed which tend to separate the bond between the metallic base and the porcelain coatings. This effect, commonly referred to in the art as "gassing" not only mechanically weakens the structure of the restoration, but also presents a cloudy and unacceptable appearance in the finished restoration.
As porcelain is transparent, it cannot be applied directly to the surface of the metallic base of the teeth. The normal practice is to apply one or more opaque coatings in the form of a water suspension of a powdered frit. Because of the difficulty in applying thick coats, it is customary to apply at least two thinner coats with a separate baking operation for each coat. The baking temperature is slightly below the temperature required to completely fire the frit, since additional coatings are subsequently applied which must bond to the opaque layers. However, such baking operations are sufficiently hot to cause some oxidation and accompanying "gassing" in non-precious metals, resulting in cracking or crazing of the applied coatings.
If the baking steps associated with the application of the opaque coatings are eliminated, the opaque coatings, upon drying, do not firmly adhere to the surface of the base metal, and the dried particles thereof are easily dislodged by contact with a tool used for applying subsequent coatings.